As it is known, the speed reducers have the purpose to reduce the rotation speed of an output shaft with respect to the rotation speed of the drive shaft. In particular the simple type epicyclic reducer trains consist of three elements, two at the end, called sun wheel or sun gear and crown wheel or ring gear, and a train-carrier free to rotate around a common axis. Such reducers usually include: a retaining cylindrical housing provided with a teeth made on its internal circumferential surface, a plurality of planetary gear wheels, generally three spaced 120° each other with axis parallel to the output axis, and idle on the respective pins integral with the train-carrier in order to be engaged with the teeth of the retaining housing.
Moreover the known reducer include a toothed sun wheel coupled to an input shaft engaging the planetary wheels.
The known reducers allow a direct motorization, high precision and power and an only radial directed stress strongly attenuated by the mesh in three (or more) points, and the limits of reduction ratio are overcome with solutions providing two (or more) trains in cascade.
A disadvantage of these known reducers consists in the lengthening of the sun gear wheel on an average of ⅓ with respect to the normal ratio module/length of the teeth and in the increasing of the torque and deflection stresses which determine noisiness and efficiency loss.